The escalating requirements for high densification and performance associated with ultra large scale integration semiconductor device require design features of 0.25 microns and under, such as 0.18 microns and under, increased transistor and circuit speeds, high reliability and increased manufacturing throughput. The reduction of design features to 0.25 microns and under challenges the limitations of conventional semiconductor manufacturing techniques.
As design features shrink to less than about 0.25 micron, it is necessary to significantly reduce the depth of the source and drain regions below the surface of the semiconductor substrate, particularly the lightly doped source/drain regions, i.e., the junction depth (X.sub.J). For example, in forming a polycrystalline silicon gate having a width of about 0.25 microns, the junction depth (X.sub.J) should not exceed about 800 .ANG.. However, the formation of a shallow X.sub.J less than about 800 .ANG. employing conventional semiconductor manufacturing methodology is problematic.
Conventional methodology comprises implanting boron ions into regions of a crystalline silicon semiconductor substrate to form P-type source/drain regions. The boron ions are implanted at an energy selected to determine the eventual X.sub.J, and at a dosage selected to control the desired concentration. As boron is an extremely light element, it must be implanted at a very low energy in order to achieve a shallow X.sub.J. Accordingly, boron is typically implanted at an energy of about 5 KeV.
It has been found, however, that upon subsequent activation annealing, boron diffusion into the crystalline silicon layer proceeds apace, such that the junction depth of boron exceeds the target X.sub.J of no greater than about 800 .ANG.. The problem of undefined X.sub.J is believed to stem from various factors. For example, boron implantation is believed to damage the monocrystalline silicon substrate generating interstitial atoms of silicon, i.e., silicon atoms that are displaced from the monocrystalline lattice to occupy spaces between silicon atoms in the monocrystalline lattice. During the high temperature activation anneal, boron diffuses into the monocrystalline silicon layer by attaching to the generated interstitial silicon atoms, causing an extremely rapid diffusion of boron into the monocrystalline silicon layer. Such a rapid boron diffusion causes the dopant profile and, hence, X.sub.J, to extend below the targeted maximum of 800 .ANG., i.e., to a X.sub.J of about 2000 .ANG. or deeper, notwithstanding the low initial implantation energy of about 5 KeV.
A previous approach to the undefined X.sub.J problem is disclosed in copending application Ser. No. 08/726,113 now U.S. Pat. No. 6,008,098, and comprises initially forming a surface amorphous region in the monocrystalline silicon substrate. The surface amorphous region is formed by ion implanting appropriate neutral impurities, such as germanium or silicon. Boron is then ion implanted into the essentially amorphous silicon region which does not contain any appreciable amount of interstitial silicon atoms to which boron would otherwise attach. Accordingly, upon subsequent activation annealing, rapid diffusion of boron by transient enhanced diffusion is not significant due to the substantial lack of interstitial silicon atoms and, hence, the X.sub.J can be controlled by the appropriate selection of the implantation energy. During activation annealing, the amorphous region is crystallized by solid phase epitaxy.
It was found, however, that end-of-range damage, e.g., defects, comprising dislocations and stacking faults, occurs upon crystallization of the surface amorphous region during activation annealing. Such end-of-range defects are present in a subsequently formed depletion layer resulting in leakage. Accordingly, there exists a need for a method of manufacturing a semiconductor device comprising a shallow X.sub.J employing the amorphization technique while avoiding the generation of end-of-range defects upon crystallization of the surface amorphous region.